The present invention relates to an input protection circuit for an electro-optical device such as an active matrix liquid crystal display device provided with nonlinear elements for respective picture elements.
An example of the conventional electro-optical device is shown in FIG. 2 in which an shows equivalent circuit of an active matrix liquid crystal display device utilizing nonlinear resistive elements of the two-terminal type is shown. The device shown in FIG. 2 includes a row electrode group 11 and a column electrode group 12, each containing normally 100 to 1000 electrodes. A pair of a liquid crystal cell 13 and a nonlinear element 14 are formed in series at each intersection of the row electrodes and the column electrodes. When a voltage is applied across the pair to drive the liquid crystal cell 13, the nonlinear element 14, having a specific equivalent resistance and equivalent capacitance, undergoes abrupt change of its resistance so that the liquid crystal cell, 13 exibits considerably sharp turning-on characteristics as compared to the case where the liquid crystal cell is driven alone. FIG. 3A is a structural diagram of a substrate formed with a picture element containing the nonlinear resistive element of FIG. 2, and FIG. 3B is a structural section taken along line A--A' of FIG. 3A. The nonlinear resistive element is comprised of a silicon nitride film 16 composed of a compound of silicon and nitrogen and having nonlinear current-voltage characteristics, the element is formed on a transparent picture element electrode 15 made of ITO(indium tin oxide) etc, under a row electrode 17 comprised of a metal film disposed on a glass substrate 18.
FIG. 4 shows the current-voltage characteristic of the nonlinear resistive element disposed between the transparent picture element electrode 15 and the row electrode 17. In FIG. 4, the abscissa indicates applied voltage and the ordinate indicates current flowing into the element in logarithmic terms. The graph shows the sharp current-voltage characteristic. The row electrode 17 is floated with respect to the earth potential. Therefore, when electrostatic charge is applied to the electrode in the order of several thousands volts, the voltage is imposed directly on the nonlinear element to thereby destroy the element which has electrostatic withstanding voltage of 20 through 50 V. Thus, it is difficult to maintain the production yield rate and ensure the reliability of device.